Often, it is desirable to modify software applications. For instance, where the software application is faulty in some way, i.e., contains one or more “bugs” that interfere with the intended operation of the application, it is desirable to modify the application code to repair the application. Normally, application code may be modified with a software patch that comprises replacement code that is used to replace the original code. Although it is not necessarily difficult to patch software in the manner described above, such patching is static. Specifically, the software patch must be developed off-line and installed while the application is not running. This can create problems where the application is one that must run continuously, e.g., network server applications, financial transaction applications, telephone switching applications, airline reservation and air traffic control system applications, etc. When it comes to such applications, the user must be able to upgrade the software to fix bugs, improve performance, expand functionality, and so forth. In the simplest case, upgrades and bug fixes require the system to be shut down, updated, and then brought back on-line. This, of course, is not acceptable for non-stop applications and, at best, will result in loss of service and revenue.
At present, there are several solutions to the problem of updating code of a non-stop application. One approach is to provide application-specific software support in conjunction with redundant hardware (already present to support fault tolerance) to enable so-called hot standbys. Clearly, this approach is expensive and, perhaps worse, adds to the complexity of building applications. Much of the complexity comes from the need for the standby machine(s) to keep or gain the state maintained by the running application. Less sophisticated users may not have the resources necessary for such a solution and seek simpler, more general, solutions.
Although dynamic updating methods have been suggested that do not require redundant hardware, no prior general-purpose methodology achieves a practical balance between flexibility, robustness, low overhead, and ease of use. For example, while dynamic linking may facilitate the addition of new code to a running program, it cannot be used to replace existing bindings with new ones. In addition, those systems that do allow replacement typically either limit what can be updated or how the updates may occur. These limitations leave open the possibility that a software update may be needed yet cannot be accomplished without downtime. Furthermore, there are few safeguards to ensure update correctness. Some systems, for example, require potentially error-prone hand-generation of complex patch files. Others rely on uncommon source languages or properties and hence are not broadly applicable. Furthermore, some systems impose a high overhead, either due to implementation complexities or due to a reliance on interpreted code.
From the foregoing, it can be appreciated that it would be desirable to have a system and method for dynamically replacing application code without interrupting application operation.